Facebook friend-finding tools -

• Friend Finder runs a check of the emails in your address book to see if any of the addresses match people who currently have profiles on Facebook.
• Classmates Search helps you search by high school, or by college.  You can make your search more specific if you add the year of graduation.
• Coworkers Search helps you find people you know from your current or former job.
• Name Finder helps you find people by name.
• Facebook will also suggest new friends based on their relationship to existing friends, schools, employers or regional networks. 

If that process yields too little or too much information to be useful, or if your friend is not already on Facebook, you still have resources to help you find them.  For instance,

• If you know the name of the person you are looking for, you can use Google to search for them.   Simply go to Google and enter their name.  If your friend belongs to any of the several dozen social networking sites (see Wikipedia for a complete list of such sites), or has a web page, or has any other online presence, their information will show up in the search results.  Once you have their email address, you can invite them to join you on Facebook.
• If that doesn't work, you can contact friends of friends to see if they know how to find the person you are looking for.